1. Field of the Invention
This invention relates to the detection of the presence of Benzene, Toluene and Xylene (BTX) in surface and ground waters and adjacent soils. The absence of said hydrocarbons relating to underground fuel storage tanks must be demonstrated by requirement of Federal and State law, by the operators of said tanks.
2. Description of Prior Art
Heretofore the absence or presence of BTX in surface and ground waters and adjacent soils was demonstrated by leak testing of adjacent underground fuel storage tanks. This leak determination method employed filling the tank being tested with fuel and measuring the fuel subsidence, as measured by viewing the fuel level change in a graduated glass column, over a time interval. Variance due to fluid temperature change and air entrapment is compensated for by computation and operator skill.
Leak determination of underground fuel storage tanks is also achieved by inventory reconciliation methods. In such methods the operator balances the volume of fuel in, versus the volume of fuel out and cmpares ending inventories with starting inventories. A computer variant of the above checks storage tank fuel level for subsidence during closed hours, while correcting for temperature changes in the fuel. Fuel lines from submersible pumps to dispensers, are checked for leakage, by a valve arrangement that terminates fuel flow if expected fuel line pressure is not achieved.
Drilling test holes adjacent to underground fuel storage tanks and sampling the adjacent soil and water for the BTX components of fuel is used in some situations. A variance of the above, is to case the sample hole with casing at the top and perforations at the bottom of the casing and monitor for BTX vapors.
New installations utilize double wall underground tanks and double wall flow lines to contain any leak. Instrumentation sensing BTX vapors signal a leak in the inner wall of the tank. Also loss of vacuum in the volume between the underground tanks inner and outer walls is used to indicate a leak, in some designs.
A disclosed leak detection method also describes a grid of low voltage wires that change resistance when exposed to BTX fluid and the resistance change is used to signal a leak.
Determination of tank leakage by observation of the subsidence of fuel in a glass column has been proven by EPA tests to have a larger error factor than the leak rate to be determined. Additionally this test is usually performed only annually and any leak that developed immediately thereafter would be undetected for nearly a year. In this probable scenario the tank operator would face a very large clean-up expense.
Determination of leakage by inventory reconciliation faces the same problem of accuracy of measurement. Usually fluid level is determined by manually sticking the tank with a wooden stick graduated in inches, subdivided to quarter of inches. Any settling of the tank from level or operator inconsistencies will affect accuracy. Although computerizing the above test method and using, fuel height, fuel level and temperature sensors does improve probably accuracy, small leaks may not be detected.
Drilling the annually test holes and sampling surface and ground waters and soils for BTX, cannot be disputed for accuracy. Economy precludes performing these tests more than annually, therefore, this method suffers because it does not detect a leak that occurs in the interval between tests.
Improving the method above involves drilling permanent monitor wells and placing BTX vapor sensors in the vadose zone adjacent to the underground tanks to be monitored. The reliability of vapor detection for leak determination of BTX fluids is clearly shown in the study conducted by Campo/Miller and presented in the report entitled : "Test report for evaluation of BTX (Naptha) migration patterns through various soils when discharged from leaking underground storage tanks", published by Campo/Miller.
Currently several manufacturers offer instrumentation for sale that places a sensor in the monitor well bore, said sensor is connected by wiring to electronics that perform the test for BTX vapors. Usual installation involves drilling and completing monitor wells, and placing at remote locations electronics connected by buried wiring to a sensor in the monitor well bore. The cost of tearing up concrete and asphalt paving to lay the signal line has prohibited this approach from broad use, particularly in existing tank installations. New installations with double wall tanks are not as greatly affected by cost increases due to having to run signal lines from the down hole sensor, in this case the sensor conduit is vented to the volume between the tanks inner and outer walls. However, having to run underground signal lines can be a cost deterent to use in large pipe line and tank farm installations.
In its optimum form a completely self contained BTX vapor sensor, intrinsically safe, with the ability to signal the presence of BTX vapors in a monitor well or a double wall storage tank would have great advantages over the current state-of-art. Such a device would fit into a two inch or smaller diameter pipe and signal leakage by visual, audible, radio or telephonic means. The scope of the invention presented herein delineates such a device.